Move predicates into own package

Since they're useful in many places

@gaborcsardi do you already have a mini package for predicates? In purrr, we have:

``` r
is_atomic <- function(x) {
  typeof(x) %in% c("logical", "integer", "double", "complex", "character", "raw")
}
is_vector <- function(x) {
  is_atomic(x) || is.list(x)
}
is_bare_list <- function(x) {
  !is.object(x) && is_list(x)
}
```

etc
